1. Field of the Invention
The present invention relates to an ultrasound enhanced drug delivery apparatus, and more particularly, to an ultrasound element which can be movably positioned within a drug delivery sheath.
2. Description of the Related Art
Thrombus formation is a protective and healing mechanism, however, formation of thrombi can be detrimental. For instance, if a blood vessel becomes blocked, distal tissue may be deprived of oxygen with resulting damage or necrosis. In the case of cerebral circulation, an arterial thrombus blockage is one cause of cerebral strokes. In the case of coronary thrombosis, blockage and subsequent distal tissue necrosis of cardiac muscle tissue will impair cardiac pump output, may cause electrical abnormalities, and potentially catastrophic heart failure and death. The thrombus can form at the site of artery narrowing due to arterial wall damage or disease, or the thrombus may have broken free from some proximal site only to become wedged in a distal stenosis. Thrombus can also form subsequent to attempts to remove a stenosis using balloon angioplasty or rotary atherectomy.
Ultrasound sheaths have been described specifically for removal or dissolution of thrombus (U.S. Patents: Tachibana U.S. Pat. No. 5,197,946; Bernstein U.S. Pat. No. 5,163,421; Weng U.S. Pat. No. 5,269,297). The sheaths of Bernstein and Weng place an ultrasound generator external to the body and transmit acoustic energy through a metal wire wave-guide to the distal sheath. The sheath of Tachibana includes a small ultrasound element positioned at the distal end of the sheath that is energized by electrical wires. In either case, ultrasound energy is delivered to and radiated from the distal tip of the sheath in the vicinity of a blocking thrombus. The application of ultrasound can directly emulsify nearby thrombus through the motion of the sheath tip, associated cavitation, and bioeffects.
The application of ultrasound can also enhance delivery of drug into a vessel wall. There are instances where the vessel wall is diseased or has been injured during balloon angioplasty or rotary atherectomy. Narrowing of the vessel can occur in response to these injuries. Certain drugs, such as heparin, may inhibit this narrowing of the blood vessel if the drug can be delivered into the blood vessel wall. A sheath can be used to deliver drugs into any portion of the body or target organ. Ultrasound energy in the presence of these drugs can enhance the delivery through and across bodily fluids and tissue. Hence, an ultrasound drug delivery sheath placed in a blood vessel will assist delivery across the blood vessel Wall, whether it be an artery or a vein, into the surrounding muscle or tissue.
The intensity of the ultrasound delivered from a cylindrical ultrasound element decreases exponentially with radial distance from the sheath tip. Hence, treatment of thrombi is limited to the circumferential area surrounding of the sheath tip of a sheath with an ultrasound element. This limited treatment area may be effective for small length clots, however, larger clots must be treated one section at a time.
Some thrombi can be large. For instance, a deep vein thrombus in a patient's lower leg and can have a length from several centimeters to as much as 30-50 cm long. Early treatment protocols for these long thrombi used a drug infusion sheath to drip lytic drug at one end of a thrombus. As the thrombus was dissolved, the sheath would be advanced. This process was repeated until the entire clot was dissolved. More current therapy for a deep vein thrombosis is to use an infusion sheath with drug infusion ports distributed along the lateral dimension of the sheath. The sheath can be pushed through the entire length of the clot. The thrombolytic drug is then infused throughout the lesion for a period of hours.
There is a need for an ultrasound sheath that is useful for treating a deep vein thrombus to enhance and accelerate the action of the thrombolytic drug. There is a further need for an ultrasound sheath that is useful for treating vessel lesions, particularly those that have extensive lengths.